In waste water treatment, surface aerators are known. They are often used in an aerating tank of an active sludge installation and their purpose is to dissolve oxygen in the water, thus providing the micro organisms in the aerating tank with oxygen.
Transfer of oxygen mainly takes place at the interface between air and water, and an optimum transfer of oxygen of air to water is obtained by making the interface area as large as possible. Furthermore, the transfer of oxygen is promoted by keeping the oxygen content near the interface in the water as low as possible, because the higher the oxygen content of the water at the interface the more difficult it becomes to dissolve oxygen in water. Consequently, it is more important that the water near the interface be quickly renewed.
The amount of energy required for aerating constitutes the larger portion of the energy consumption of a waste water purification plant. It is of paramount importance, therefore, that the amount of oxygen dissolved in the water per energy unit (the oxygen input efficiency) is as large as possible.
The regulation of the oxygen input capacity of a surface aerator provided with a rotor which agitates the water takes place by changing the immersion depth and/or the speed of revolutions of the rotor. The immersion depth range of the surface aerator must be sufficiently large to cope with the normal variations in the water level of an aerating tank, while maintaining a high oxygen input efficiency. The speed range within which a high oxygen input efficiency is achieved should likewise be as wide as possible, so that the highest possible oxygen input efficiency is achieved, both when a large amount of oxygen is put in and when less oxygen is put in.